In “The Cake Eaters” superior acting elevates a small, overcrowded ensemble piece set in rural upstate New York into something a little deeper and truer than the mawkish disease-of-the-week movie it threatens to become.

The ailment in question, Friedreich’s ataxia, is a progressive degenerative disease of the nervous system. It impairs the speech and coordination of Georgia Kaminski (Kristen Stewart), a willful teenager determined to experience sex before she dies.

Georgia is thwarted in her carnal ambitions by her overprotective mother, Violet (Talia Balsam), a devotee of New Age quackery who (weirdly) takes nude art photos of her daughter. Georgia’s parents seem to have been largely edited out of the movie, presumably for reasons of running time.

Beagle Kimbrough (Aaron Stanford), the boy Georgia chooses to initiate her into the mysteries of sex, is a shy, reluctant youth who works in her high school cafeteria and paints in his spare time. When he comes courting, it is Georgia who seizes the initiative and pounces, but their first tryst is interrupted by the snobbish, ever-watchful Violet.

The promising directorial debut of the actress Mary Stuart Masterson — from a screenplay by Jayce Bartok, who plays Beagle’s older brother Guy — “The Cake Eaters” is a dual portrait of the Kaminskis and the Kimbroughs, two loosely connected small-town clans, one of which (the Kaminskis) is better off than the other. The Kimbrough patriarch, Easy (Bruce Dern), is a butcher still grieving over his wife’s recent death from cancer. During her illness Beagle took over as her primary caretaker when Easy could no longer handle the responsibility.

When Guy, a failed indie rocker, unexpectedly arrives home after scuffling in New York for several years, Beagle assails him for having been absent during their mother’s illness. Guy, a barely sketched character, immediately tries to rekindle a relationship with Stephanie (Miriam Shor), the girl he left behind to pursue rock ’n’ roll glory. Now married and with a family, Stephanie is drawn to him despite her still-simmering rage over his disappearance.

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Tensions are exacerbated when it comes out that Georgia’s salty grandmother Marg (Elizabeth Ashley), a woman of means, has had a long-term affair with Easy. Watching Mr. Dern and Ms. Ashley together offers the fascination of studying two high-strung thoroughbreds reacting to one another’s proximity with snorts and flashing eyes; there could be a stampede at any moment.

Ms. Stewart’s tough, strong performance avoids sentimentality to such a degree that her character isn’t even particularly likable. But as she schemes to get what she wants come hell or high water, you cheer her on.

If all this domestic turbulence is a bit too much for one small movie, “The Cake Eaters” is good at developing the subplots on which it concentrates: the uneasy relationships among the Kimbrough men, and Georgia and Beagle’s evolving connection. The movie’s broader psychological landscape, however, can leave you scratching your head. It is like watching a movie on television in a format that doesn’t fit the screen.

THE CAKE EATERS

Opens on Friday in New York and Los Angeles.

Directed by Mary Stuart Masterson; written by Jayce Bartok; director of photography, Pete Masterson; edited by Joe Landauer and Colleen Sharp; music by Duncan Sheik; production designer, David Stein; produced by Jesse Scolaro, Allen Bain, Darren Goldberg, Elisa Pugliese and Ms. Masterson; released by 57th and Irving Productions and the 7th Floor. Running time: 1 hour 26 minutes. This film is not rated.